Optical switches and modulators are widely used in optical communication systems. Such optical systems include waveguide (e.g., optical fibers, planar wafer-based circuits) and free-space systems, or combinations of such systems. In many applications, modulation in optical communication systems is implemented as digital modulation in which the modulator is in either an ON or OFF state, as in an optical switch. Thus, optical modulators in digital signaling systems are essentially ON/OFF optical switches.
One approach to optical switching and modulation is based on controlling the phase of portions of an optical signal to selectively control the interference between these portions. For example, in one approach, the input optical signal is split into two matched portions, then the phase difference between them is controlled, and then the portions are recombined to form the output signal. If the phase difference is 180 degrees (or some multiple thereof), then completely destructive interference occurs when the portions are recombined, resulting in output signal ideally having an intensity of zero. This configuration can be used as the OFF state of the switch or modulator. Conversely, if the phase difference is zero degrees (or some multiple of 360 degrees), then completely constructive interference occurs, resulting in the output signal having essentially the same intensity as that of the input signal. This configuration can be used as the ON state of the switch or modulator.
There are several approaches to causing the phase difference between the portions of the input optical signal. One approach is to cause a difference in refractive index of the media in which the portions are propagating, which in turn will cause a phase difference between the portions. Various electro-optic, thermo-optic and stress/strain-optic mechanisms can be used to vary the refractive index of a propagation medium.
One of the important parameters of optical switches and modulators is the power required by the optical switch or modulator to switch between ON and OFF states (i.e., the drive power requirement). In most applications, it is desirable to reduce the drive power requirements of the optical switches and modulators.